Acoustic resonators, and particularly Bulk Acoustic Wave (BAW) resonators, are used in many high frequency communication applications. In particular, BAW resonators are often employed in filter networks that operate at frequencies above 1.5 GHz and require a flat passband, have exceptionally steep filter skirts and squared shoulders at the upper and lower ends of the passband, and provide excellent rejection outside of the passband. BAW-based filters also have relatively low insertion loss, tend to decrease in size as the frequency of operation increases, and are relatively stable over wide temperature ranges. As such, BAW-based filters are the filter of choice for many 3rd Generation (3G) and 4th Generation (4G) wireless devices, and are destined to dominate filter applications for 5th Generation (5G) wireless devices. Most of these wireless devices support cellular, wireless fidelity (Wi-Fi), Bluetooth, and/or near field communications on the same wireless device, and as such, pose extremely challenging filtering demands. While these demands keep raising the complexity of the wireless devices, there is a constant need to improve the performance of BAW resonators and BAW-based filters as well as decrease the cost and size associated therewith.
Electrostatic interference in a BAW filter (e.g., BAW die, BAW filter die, etc.), or other BAW structure, can negatively affect performance. In particular, parasitic capacitance (i.e., capacitive parasitics, parasitic coupling, parasitic capacitance coupling, etc.) in a BAW filter can degrade filter rejection in a stopband. Types of unwanted parasitic capacitance include inter-stage coupling and capacitive feedthrough. Capacitive feedthrough is generally more severe than inter-stage coupling and is defined by a parasitic capacitance coupling directly between the input and output of the filter. Feedthrough capacitances as little as 0.5 F can cause significant filter rejection degradation, thereby negatively affecting filter performance.